In the domain prefix field, you'll want to choose either the name of your business or your domain name without the .com. For example if my domain is office365test.com, I would choose office365test as my domain prefix. What this does is creates a primary administrative username for your Office 365 account. This account is the global administrator of all the Office 365 licenses. That user will be domainprefix.onmicrosoft.com. In our example, my primary administrative user would be office365test.onmicrosoft.com

*If a domain prefix with your extension already exists, you will receive an error indicating this and you'll need to choose another domain prefix.

6) Once you get a success message, you will be emailed your Office 365 password to the email on file for your account. Once you receive this email, you'll visit https://login.microsoftonline.com and type in the primary administrative user (domainprefix.onmicrosoft.com) and the random password you were emailed.